Various services in the Telecom network rely on a low latency variation transfer to ensure proper operation. Such services among others include synchronization and multi-media services (voice and video). Currently, with the prevalence of layer-3 and layer-2 packet networks, such as Internet and Ethernet, cost can be reduced by transmitting timing synchronization and voice service, as well as distributing video service between telecom equipment within the packet network. However, due to the store-and-forward operation of packet networks, those services will experience an uncertain delay, which will affect the accuracy of in-time delivery. This uncertainty in delay is commonly referred to as packet delay variation (PDV).
In addition to its effect on timing synchronization signals, PDV also has a significant effect on packets transferring voice and video data. It is desirable for packets transferring voice data to have a minimum, and above all, a well controlled, latency or delay, thus a substantial reduction or elimination of PDV is required. However, the amount of latency or delay does not carry a similar importance, since it is more important for the latency or delay to remain constant throughout the transfer. Therefore, it is desirable to at least significantly reduce or eliminate PDV for both timing synchronization signals and data packets in packet transfer networks.
Previous attempts to solve the problem of PDV and provide a predictable latency have included best effort forwarding in conjunction with traffic engineering as well as protocol specific manipulation of time-stamps. However, the traffic engineering of network nodes results in under-utilization of an expensive node.
Additional attempts have included work-conserving and non-work-conserving scheduling at an egress port upon reception of transferred packets. Work-conserving scheduling selects and schedules egression as soon as a previous egression is completed, while non-work-conserving scheduling selectively provides egression breaks for short, controlled periods of time. However, such scheduling methods have failed to involve real-time knowledge regarding the exact point in time when a packet would need to leave the device based on its arrival time. Thus, the scheduling methods fail to provide a jitter-bound behavior.
Regarding time-stamp-based packet synchronization methods, such as, for example, IEEE 1588, two techniques for providing timing synchronization with reduced PDV are known. These techniques include probabilistic filtering algorithms and long time averaging.
A probabilistic filtering algorithm filters out any large packet delays and uses the small packet delays for calculation of correction factors and timing synchronization of a local clock of a receive node with a transmit clock of a transmit node. This method usually has high complexity. The process of obtaining satisfactory small delays for calculation is random, therefore, successful performance of the probabilistic filtering algorithms in the short-term is not guaranteed.
Long time averaging provides a method that averages the delay of multiple transferred packets in order to eliminate the PDV. However, in an actual system, obtaining an absolute time is difficult because the local clock of the receive node is not accurate and requires synchronization. Any adjustment of the local clock may adversely affect the result of the delay averaging. Moreover, this method is not flexible, in that the result of the averaging cannot be controlled.
For a multitude of network services, such as, for example timing over packet (ToP) services, it is desirable to achieve a deterministic per-hop behavior that defines the policy and priority applied to a packet when traversing a hop. More specifically, the deterministic per-hop behavior may be considered a determination of a predictable latency at network elements, such as, for example, switches, routers or multiplexers, which may be particularly useful for timing synchronization over packet networks.